1. Field of the Invention
The present invention generally relates to a phosphate base laser glass, and, more particularly, it relates to a phosphate base laser glass possessing a high induced emission cross section, a low non-linear refractive index coefficient, and excellent acid resistance and devitrification resistance.
2. Description of the Prior Art
Laser glasses for nuclear fusion have recently been required to have a high amplification factor and a low non-linear refractive index coefficient (the refractive index coefficient proportional to the square of the intensity of the electric field applied to the laser).
The amplification factor G of a laser is expressed by the equation EQU G=e.sup.N.multidot..sigma.
wherein N is the inversion density and .sigma. is the induced emission cross section. As is shown in the above equation, in order to increase the amplification factor G, the value of N.multidot..sigma. is required to be large, and, further, since the inversion density is determined by the input energy of a xenon flash lamp to the laser, it is desired as a characteristic of a laser glass that the induced emission cross section of the laser glass be as high as possible.
The refractive index of a laser in a high electric field is shown by the equation EQU n=n.sub.o +n.sub.2 E.sup.2
wherein n.sub.2 is the non-linear refractive index coefficient, E is the intensity of the applied electric field and n.sub.o is the refractive index coefficient in a non-laser light (non-electric) field. If the non-linear refractive index coefficient is high, the laser light undergoes self-focusing to break the laser glass which makes the further use thereof difficult. Therefore, it is desired that a laser glass have a non-linear refractive index coefficient which is as low as possible. Phrased differently the lower the value of n.sub.2, the stronger the power of the laser light can be. Generally, it can be used at a several G W/cm.sup.2 for a nanosecond laser pulse and at a several hundred G W/cm.sup.2 for a picosecond laser pulse. When n.sub.2 is as low as possible, the laser glass does not break even when the upper limit of the power is high. However, in conventional silicate base laser glasses .sigma. is 2.5-3.0.times.10.sup.-20 cm.sup.2 and n.sub.2 is 1.5-2.0.times.10.sup.-13 e.s.u., and in conventional phosphate base laser glasses .sigma. is 3.0-4.0.times.10.sup.-20 cm.sup.2 and n.sub.2 is 1.5-2.0.times.10.sup.-13 e.s.u.
The laser glass disclosed in Japanese Patent Application Laid Open No. 114,615/'74 is a P.sub.2 O.sub.5 -alkali metal oxide-rare earth element oxide series laser glass containing a high proportion of rare earth element oxides, and the non-linear refractive index coefficient thereof is as high as 1.8-3.0.times.10.sup.-13 e.s.u. When used as a laser glass, the laser light causes self-focusing to break the laser glass and hence makes the further use thereof difficult. On the other hand, when rare earth element oxides in the laser glass are replaced with Al.sub.2 O.sub.3, the non-linear refractive index coefficient thereof becomes as high as 1.2-1.4.times.10.sup.-13 e.s.u., and thus breakage of the laser glass will not occur.
The present applicants previously filed a patent application on a P.sub.2 O.sub.5 -Li.sub.2 O-Na.sub.2 O-Al.sub.2 O.sub.3 series laser glass (Japanese Patent Application No. 134,788/'74). While this laser glass has a low non-linear refractive index coefficient n.sub.2, it does not have excellent acid resistance and devitrification resistance due to the large proportions of Li.sub.2 O and Na.sub.2 O therein, which permits the utilization thereof as a laser rod.